Large-scale grids and other federations of distributed resources that aggregate and share resources over wide-area networks present major new challenges for scientists. The book focuses on the challenge to enable scalable, high-level, online simulation of applications, middleware, resources and networks to support scientific and systematic study of Grid applications and environments. Its contributions are on the intelligent control of distributed systems where simulation is used in decision making as a way to predict future performance under some control law in question. We describe the creation of a software tool that is designed to facilitate the modeling, simulation and control of distributed systems. We present methods and techniques to be applied in the design and implementation of a generic modeling framework. We also present results obtained using the developed simulator that are of important value in the decision-making process of the deployment of real-world distributed system technologies.